Diffractive optical system with synthetic opening and laser cutting device incorporating this system

ABSTRACT

An optical device for focusing a light beam. The device includes a Fourier diffractive element that can separate an incident beam into n beams along n directions which are symmetric about an optical axis. The device also includes a diffractive element including n Fresnel lenses capable of refocusing the n beams onto the optical axis. The device may be used with lasers and laser cutting devices.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the domain of laser cutting, particularly inview of dismantling and/or disassembly operations.

2. Discussion of the Background

The advantage of using laser techniques for carrying out dismantlingoperations is that it provides some process flexibility (possibility ofremote cutting and variation of the cutting distance), and largepotential benefits in terms of secondary cutting waste (less aerosols,less swarf, smaller cutting widths).

However, very few of these systems have been used, for three mainreasons:

the most powerful laser sources, particularly CO₂ sources, require beamtransport by mirrors since the wave length of CO₂ sources isincompatible with materials used for power optic fibers. These mirrorsenable the necessary movements for cutting. The resulting mechanism isvery complicated, particularly when it is necessary to crossconfinements and biological shielding in nuclear installations.

the power of sources producing a beam that can be transported by opticalfibers (for example Nd:YAG) did not exceed one kW until the last fewyears. Furthermore, the use of the optical fiber transmission techniquecauses problems which will be described below,

traditional laser cutting processes require the use of assistance gasesto make cutting feasible (flushing of molten material, protection ofoptics), which consequently requires that the part to be cut should befollowed very closely (at about 1 to 2 mm).

These constraints have made it very difficult to use lasers in cuttingfor dismantling for which confinement is compulsory, it is oftennecessary to pass through biological shielding, and proximity followingis practically impossible since the geometry of objects to be cut iscomplex, not well known (since it is often difficult to measure,particularly in a nuclear environment) and very variable.

The use of optical fibers has also created a number of problems.

The emerging beam is disturbed due to its path, particularly at the exitfrom the fiber; for example, a beam with a Gaussian distribution aboutits center line has an annular shape at the exit from a conventionalstep-index fiber. Consequently, the maximum energy distribution iseccentric, and cutting performances are reduced.

For focal length of the order of one meter, beam disturbances can becorrected using conventional glass lenses, for example type BK7. Theselenses are transparent to infrared (wave length 1.06 μm) emitted by anNd:YAG laser. But the weight of a device based on this technology andfor this focal length is around 15 to 20 kg. Furthermore, this type ofassembly is as delicate as a telescope with the same aperture (20 cm)and, for example, must be protected from shocks that would modify thesettings and affect operational safety.

Consequently, remote servocontrol of the focal length (zoom) necessaryfor cutting parts with complex geometry is practically impossible usingconventional glass mirrors or lenses with reasonable size, fragility andweight parameters. Therefore, it is not really feasible to achieve focallengths exceeding a meter, for laser cutting for dismantling usingconventional optical systems that are more suitable for use in thelaboratory and/or for short focal lengths (less than one meter) than forcutting on nuclear sites at long distances.

The use of mirrors to manufacture focusing lenses would further increasethe size for an equivalent weight and fragility, taking accountparticularly of stiffening devices and mirror mountings.

SUMMARY OF THE INVENTION

The first objective of the invention is an optical focusing device withlimited size and weight, which is fairly robust and is compatible withthe use of a power laser, particularly for use in a laser cuttingdevice. The optical focusing device must enable correction of defectscaused by an optical laser beam transmission fiber.

The term “Fourier diffractive element” used below refers to adiffractive element that diffracts and divides an incident planewaveinto n planewaves. As is well known in the art, the mathematicalrelation between the complex amplitude transmission of a far fielddiffractive element and its diffraction pattern results from a Fouriertransformation. Furthermore and as discussed below, the designalgorithms for the diffractive element includes an iterative Fouriertransformation. Consequently, persons of ordinary skill in the art oftenrefer to such diffractive elements as “Fourier diffractive elements.”

More precisely, the first purpose of the invention is an optical laserbeam focusing device, comprising:

a Fourier diffractive element that is capable of separating an incidentbeam into n beams along n directions symmetric about an optical axis ofthe device,

a diffractive element comprising n Fresnel lenses, capable of refocusingthe n beams on the optical axis.

The advantages related to this device include the following.

The diffractive optical system corrects beam aberrations resulting fromthe fact that the beam was transported by an optic fiber. Thiscorrection may be made using a Fourier lens or Fresnel lenses, orpartially corrected using both types of lenses.

This device is capable of synthesizing a large aperture using smallcomponents. It is a Fourier-Fresnel type assembly with a syntheticaperture, which separates the incident beam into n identical beams(using a Fourier diffractive element) and focuses these n beam portions(using n Fresnel lenses) and consequently replaces a lens with a largeaperture, and therefore a large size in general, by a smaller assembly.

From the manufacturing point of view, the problem of manufacturing alarge Fresnel diffractive component (100 mm minimal diameter) is reducedto the problem of machining n smaller components.

Furthermore, it is easy to use this type of device. The focal length isvaried simply by moving one of the components parallel to the other.Since the chosen Fresnel-Fourier formula is not critical, theirrespective positions may be controlled with mechanically simple means.

This type of device is adapted to use with a power laser. Furthermore,it forms an optical component which is not very sensitive toenvironmental disturbances (vibrations, shocks, dust, etc.) and istherefore suitable for use on site, for example for a laser dismantlinginstallation. Finally, this device may easily be replaced, for examplein the form of modules; no critical optical positioning or realignmentis necessary during the replacement.

The two elements (Fourier diffractive element and all Fresnel elements)may be laid out such that the device operates in transmission or inreflection. In the latter case, the two elements are laid out such thatan incident beam is reflected by the Fourier diffractive element andbroken down into n beams, each of these beams then being reflected byone of the Fresnel lenses towards the focus point.

Means, for example such as refractive elements, may also be provided forcollimating the incident beam.

Another purpose of the invention is a device for creating focused laserradiation, comprising means of generating a laser beam and a focusingdevice like that described above.

The result is an assembly that operates in a fairly simple manner, withall the advantages described above in relation to the focusing device.

In particular, if the beam is transmitted by optical fiber at the exitfrom the laser, the focusing device can correct aberrations related totransport by optical fiber.

Finally, another purpose of the invention is a device for laser cutting,comprising a device for generating focused laser radiation as describedabove.

This focusing lens can be built into a device that can be used on acutting site. Focal lengths of the order of one meter (or more) mayeasily be achieved, which is impossible with conventional opticalsystems (refractive components). The “zoom” function can be made in avery simple manner, and does not require the use of large and fragileglass mirrors and/or lenses; all that is necessary to effectively changethe focal length is to displace one of the optical components withrespect to the other.

Furthermore, this process does not use any make up gas projection nearthe laser beam impact area.

Means may be provided to trigger laser emission means, in a pulsed andrelaxed manner.

The use of a laser operating in pulse mode makes it possible to createvery high energy plasma close to the impact area on the part to be cut.The consequence of pulse mode is to add a flush effect to heating of thematerial in order to replace the assistance gas used in conventionalindustrial applications. This flush effect is explained by the qualityof the very high energy plasma created by laser pulses.

This plasma has another positive effect on cutting; it enables selffocusing of the beam within the thickness of the cut material. This isexplained by the fact that very deep high quality cuts are obtaineddespite the absence of assistance gas (cuts are very thin with paralleledges and the size is practically the same as the diameter of the focusspot, and with parallel edges) (thick materials can be cut because ofthe depth of the cut). The fact that thin cuts can be obtained alsoreduces the secondary waste (such as gas or metal ball type sediments)produced, and therefore prevents the focusing means present at the exitfrom the optical fiber becoming dirty too quickly.

Finally, also because of the creation of a plasma, operation in pulsemode enables a greater tolerance on other cutting parameters; forexample, the precision of focusing is not very critical, and in any caseis less critical than in techniques using a continuous beam. Plasma canproduce an energy density higher than continuous mode. Due to thistolerance there is less need for servocontrol of focusing, and in-depthcuts can be made by self-focusing of the beam.

The use of a single laser mode (relaxed mode) can give a modal stabilitywhich guarantees uniform coupling for cutting.

It becomes easy to control a laser in pulse mode; the height, width andspacing of pulses are adjustable. Therefore it is possible to optimizecutting operations as a function of the context, given the effect ofthese pulse parameters on the space/time physics of the plasma createdclose to the impact area. For example, it is not necessary to send veryhigh energy pulses if cutting is done on small thicknesses.

The end of the optical fiber may form part of a cutting head.

Means can also be provided for controlling the position of the end ofthe cutting head and/or means of controlling the beam focus.

Furthermore, means may also be provided for moving end of the cuttinghead. Thus, the cutting head itself will be moved in the case of a partto be cut which cannot be moved. These means of displacement may forexample comprise one or several robot controlled arms.

BRIEF DESCRIPTION OF THE DRAWINGS

In any case, the characteristics and advantages of the invention will bebetter understood after reading the following description. Thisdescription applies to example embodiments given for explanatorypurposes and in no way restrictive, with reference to the drawings inthe appendix in which:

FIGS. 1A and 1B represent focusing devices according to the invention,in a transmission embodiment, showing a side view and a front viewrespectively,

FIG. 2 shows a focusing device according to the invention, in anembodiment, in reflection,

FIG. 3 diagramatically shows different orders diffracted by the twoelements of a device according to the invention, in reflection,

FIG. 4 is a diagram showing the steps in the calculation of a Fourierdiffractive element,

FIGS. 5A-5C show the steps necessary to make a chromium mask,

FIGS. 6A-6D show the steps involved in photolithographic transfer andmachining of a substrate,

FIGS. 7A-7D show the steps necessary to make elements with 2 ^(n) phaselevels,

FIG. 8 is a plate showing a Fresnel element and FIG. 9 is a plateshowing a portion of a Fourier diffractive element, used in a deviceaccording to the invention,

FIG. 10 is a diagram showing a cutting device according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of an optical focusing device according to theinvention will now be described with reference to FIG. 1. The systemcomprises two diffractive elements 2, 4.

The first element is a Fourier diffractive element that is used toseparate an incident beam 6 (for example transported through an opticalfiber 8) into n identical beams (n=4, 6, 8 or 10), in n directionssymmetric about the optical axis AA′.

These n beams 10-1, . . . , 10-8 (n=8 in FIGS. 1A and 1B) are thendirected towards n Fresnel lenses 4-1, 4-2, 4-3, 4-4, 4-5, 4-6, 4-7,4-8, which in turn focus the n portions of the beam towards a point F onthe center line AA′. Therefore, globally the complete device operateslike a converging lens that refocuses the energy on the optical axis ata given distance. This distance depends on the optical assembly, andparticularly on the distance between elements 2, 4 in the device.

Therefore the focusing device according to the invention uses a Fourierdiffractive element to distribute the beam on a component operating as asynthetic aperture in diffractive mode, with the lens coded on a smallpupil reproduced identically n times. The effective resulting componentaperture exceeds the physical dimension of this component.

The input beam 6 could be collimated using diffractive type means 12,for example a spherical collimation lens operating in transmission.

Another embodiment of the invention will now be described with referenceto FIG. 2. This second embodiment operates “in reflection”. As in thecase shown in FIG. 1, a beam 6, for example output from fiber 8, is senttowards a diffractive Fourier element 14. This elements operate inreflection. Beam 6 may possibly have already passed through collimationmeans 12, for example a spherical lens operating in transmission.

The Fourier diffractive element 14 distributes light from the incidentbeam into n identical beams, in n direction symmetric about the centerline BB′. These n beams 18, 20, . . . are reflected towards the secondelement 22 in this focusing device. This second element actuallycomprises n Fresnel lenses. Each of these lenses will be capable ofprocessing one of the beams 18, 20, etc. that are obtained afterreflection on the Fourier diffractive element 14, this processing takingplace in diffraction. The n beams are then focused to the same point F′:in fact n focusing spots are obtained which will interfere with eachother.

During the successive reflections on Fourier diffractive and Fresnelelements, different orders are reflected in different directions, andonly some orders are reflected towards point F′. The energy output fromunwanted orders may be recovered and trapped in cones made of lightmaterial.

In all embodiments, the device according to the invention works globallylike a lens. But the device according to the invention is much morecompact for equivalent performances (same aperture). The center line ofthe lens thus synthesized is not coincident with the optical axis (AA′in FIG. 1A, BB′ in FIG. 2). The axis of the synthetic lens is marked byaxis SS′ in FIGS. 1A and 2: it is at a distance R_(i) from the opticalaxis of the device.

According to one example embodiment, the beam 6 to be focused is thebeam of a YAG laser (wave length=1.06 microns), it passes through acollimation lens 12 and meets a binary, square Fourier diffractiveelement 14 with side a=2 cm composed of square cells with a sidedimension of 3 microns.

The Fresnel elements 22 are composed of eight identical spherical lenseswith four phase levels. The pupil of each lens is round with diameterd=20 mm, and the center of each pupil is located at a distance D=50 mmfrom the axis BB′. The Fourier diffractive and Fresnel elements areseparated by distance Δ, along axis BB′:Δ=475.7 mm. Under theseconditions, the point F′ is separated from the plane containing Fresnelelements 22 by a distance f=1400 mm. After reflection on the Fourierelement 14, the incident beam 6 is separated into eight beams reflectedin eight symmetric directions with an angle γ=60°. Similarly, afterreflection on the Fresnel elements 22, reflection takes place towardspoint F′ with an angle β between −1.637° and −2.454° from a directionperpendicular to the plane containing the Fresnel elements 22.

In this example, the distance R_(i) is equal to about 147.1 mm. Theradius of a conventional refractive lens necessary to obtain the samefocusing and aperture characteristics would be 197.1 mm.

The Fresnel lens coded through the eight identical and symmetric pupilsoff the center line, is composed of circular fringes on these pupilswith periods varying from$\frac{\lambda \quad f}{2\left( {D - \frac{d}{2}} \right)}$

for the maximum fringes to$\frac{\lambda \quad f}{2\left( {D + \frac{d}{2}} \right)}$

for the minimum fringes, where D and d have the meanings defined above.Therefore the smallest structures etched under two phase levels willhave a side dimension of 3.58 μm, and the smallest structures etchedunder four phase levels will have a side dimension of 1.97 μm.

With these data, the reflection coefficients in the different orders areshown in FIG. 3, in which the expression O(n,m)×% represents order noutput from element m, with an efficiency of ×%. The intensity of theincoming beam is assumed to be 100%.

In general, in a focusing device according to the invention, thelongitudinal alignment tolerance of the Fourier element with respect tothe Fresnel element, is coarse. This tolerance in the example givenabove is ±0.5 mm. This is due to the fact that since a Fourierdiffractive element is used, there is invariance by translation. Thelateral alignment tolerance is usually coarse, since the beams outputfrom the Fourier diffractive element are only half the size of thepupils in the Fresnel element. The optical fiber and the collimationlens also have coarse alignment tolerances (of the order of 1 mm).However, the Fourier diffractive element is preferably kept as parallelas possible to Fresnel elements. This parallelism of the two elementsmay be obtained by adjustment, for example using micrometric screwsplaced on the back of the Fourier diffractive element.

The Fourier diffractive and Fresnel elements may be made of 1 mm thicketched quartz. The diffractive element thus obtained may be supported ona quartz substrate, which may for example be 5 mm thick. It may also becovered with a gold coating (about 50 nm), this coating facilitatingevacuation of heat absorbed under the impact of radiation.

We will now describe a process for calculating the elements. Forexample, the first Fourier diffractive element may be optimized using aniterative algorithm called the GERCHBERG-SAXTON algorithm described forexample in the article by R. W. GERCHBERG et al., untitled “A practicalalgorithm for the determination of phase from image and diffractionplane pictures”, which was published in Optik, vol. 35, No. 2, pp.237-246, 1972. This algorithm is capable of quickly calculating thephase of a complete wave function, for which the intensities in thediffraction plane and in the image plane of an optical system are known.The steps in this algorithm will now be described briefly with referenceto FIG. 4.

The imposed constraints are the intensities sampled in the image planeand in the diffraction plane. The amplitudes are proportional to thesquare root of the measured intensities. These data may be stored in thememory of a microcomputer.

In a first step 32, a random number generator generates a set of randomnumbers between π and −π, which corresponds to an initial estimate ofthe phase of the sampled image amplitudes. (In FIG. 4, reference 26denotes the step in which the intensity and amplitude in the image planeare sampled and stored, reference 28 denotes the step in which the samevalues in a diffraction plane are sampled and stored).

The random values are then multiplied by the amplitudes of the imageplane (step 34). This thus generates a complex, discrete function, forwhich the Fourier transform (step 36) is then calculated by a FastFourier Transform (FFT) algorithm. The result is a complex function,from which the phases are extracted (step 38). These phase values arecombined with amplitude values in the diffraction plane (step 40) andthe function obtained is subjected (step 42) to a fast Fouriertransformation, from which the phase is extracted (step 44). The phaseis then combined with the amplitudes of the image plan (step 34) to forma new estimate of the complex function in the image plane. The processis then repeated iteratively.

In fact, the Gerchberg-Saxton algorithm is a phase calculationalgorithm. There are two fields combined by a Fourier transform (thehologram field and the far field, composed of n symmetric points limitedby the diffraction).

The algorithm allows the phase to vary in the two fields. The resultingphase in the hologram field is then digitized in two values only (0 andπ).

This algorithm is a modeling and optimization algorithm; modeling isdone during the first loop of the algorithm and optimization duringsubsequent loops.

In the special case of the optical focusing device according to theinvention, the selected constraints are the intensities in the plane ofthe diffractive element, the divergence of the beam output from themulti-mode fiber, the constraint in the far field being the energydistribution according to the n Fresnel pupils 22.

The second element (Fresnel pupil) with synthetic aperture is givenanalytically by a calculation on a conventional commercially availablecode V type optical CAD, and is aspherical. For example, each pupil canbe represented by the following phase profile:${{\varphi \left( {x,y} \right)} = {CircR}_{i}},{R_{0}\left\{ {\frac{2\pi}{\lambda}\sqrt{x^{2} + y^{2} + f^{2} - f}} \right\}}$

where R_(i) is the distance between the center of the synthetic lens andthe center of a pupil; R₀ is the radius of a pupil, f is the focallength and λ is the wave length of the laser.

The two elements of the optical device according to the invention may bemanufactured by electron beam microlithography. Binary chromium masksare prepared by electron beams, transferred and aligned with a finalsubstrate one after the other, and then ionically machined. For example,4 chromium masks are used to make an element with 16 phase levels, witha size of about 2cm×2cm, and composed of 2048 square cells, each cellcomprising a 10μm×10μm unit. The diffraction efficiency of this type ofelement is of the order of 95%, in other words only 5% of the light islost.

FIGS. 5A to 5C represent steps in making chromium mask. A layer ofchromium 52 is deposited on a quartz substrate 50, together with a resinlayer 54 that can be attacked by an electron beam 56. After the layer 54has been etched (FIG. 5B), the chromium layer 52 is pickled, and theresin layer 54 is then pickled, by dipping in an acid bath. This thusgives the required chromium mask 58 (FIG. 5C).

FIGS. 6A to 6D show the steps in photolithographic of the binarychromium mask 58 made previously, followed by machining of the finalsubstrate, to obtain an element with two levels. A layer ofphotoresistant material 62 is deposited on a quartz substrate 60. Anultraviolet radiation beam 64 is directed towards substrate 60, passingthrough the chromium mask 58 (FIG. 6A). This thus gives aphoto-resistant layer 66 reproducing the patterns of the chromium mask58 (FIG. 6B). A beam 68 of collimated argon ions pickles the surface ofsubstrate 60 to obtain an element 70, coded on the surface with twophase levels (FIG. 6D).

To obtain higher order phase levels, the substrate 70 is covered with alayer of photoresistant material 72 (FIG. 7A). A second binary mask 74made of chromium, is formed on a substrate 76 using the techniquedescribed above with reference to FIGS. 5A-5C. The mask 74 and thesubstrate 70 are then aligned, and a collimated ultraviolet radiationbeam 78 is directed towards substrate 70, passing through the binarymask 74 (FIG. 7A). This thus gives a substrate 70, the surface of which,already coded at two levels, is covered with a photoresistant layer 80itself coded at two levels (FIG. 7B). A collimated argon ions beam 82(FIG. 7C) transfers the coding from the photoresistant layer at thesurface of element 70, which is thus coded at 4 phase levels (FIG. 7D).

In general, n binary masks are used to code an element with 2^(n) phaselevels (n>2).

Fresnel elements are made taking account of the fact that the N pupilsprecisely code the same information. Therefore, in order to code with2^(n) phase levels, N rather than nN chromium masks are prepared. Thetechniques described above with reference to FIGS. 5A-7D are used toprepare masks and for etching substrates. The first chromium mask istransferred onto a quartz substrate n times (for the n pupils) using asymmetry of revolution. After ionic machining, the second, third, n^(th)chromium masks are transferred and machined ionically, until 2^(n) phaselevels are obtained on the N elements. If 16 phase levels are made(n=4), the size of the quartz substrate used will be 6 inches ×6 inches.

In all cases, the mask aligner used is capable of aligning the variousmasks within ±0.5 μm.

From a practical point of view, the production of components accordingto the invention uses the next steps in the software which include thesteps in the Gerchberg-Saxton algorithm and the steps for preparation ofmicrolithography that have been described above.

1) Fill in a 1024 by 1024 matrix with a random phase (i.e. values takenat random between 0 and 2π) and fill in a 1024 by 1024 matrix with theintensity distribution of the incoming beam (YAG beam output from themulti-mode fiber).

2) Direct complex Fourier transformation (conventional FFT).

3) The phase information (matrix) is kept, the amplitude information(matrix) is replaced by the required amplitude distribution in the farfield (in this case, eight symmetric light spots).

4) Transformation of the inverse complex Fourier.

5) The phase information is kept and the amplitude information isreplaced by the amplitude distribution output from the multi-mode fiber.

Steps 2) to 5) are repeated n times (n=number of iterations).

6) The phase information (matrix) is corrected by thresholding at twolevels:

a) if the value of the continuous phase function obtained by theGerchberg-Saxton algorithm exceeds $\frac{\pi}{2},$

the value of 1 is assigned to the phase,

b) otherwise$\left( {{0\quad {phase}\quad {function}}\quad \leq \frac{\pi}{2}} \right),$

the value of 0 is assigned to the phase.

In the manufacturing process, 0 means no etching and 1 means etching (toa depth equal to λ/(2(n−1)). (λ=wave length, n=refractive index of thesubstrate).

7) This phase information is then formatted in the GDSII format.

8) The file is handed over to the microlithography manufacturinglaboratory.

9) The element is copied in x and y to give a total circular pupil witha diameter of 2 cm.

The focusing device according to the invention may be used incombination with means of producing a laser beam. In particular, it isadapted to a high intensity pulse beam produced by lasers, for example abeam produced by Nd:YAG lasers, or iodine-oxygen lasers. Radiationproduced by these laser sources has the advantage that it can betransmitted by optical fiber, and consequently the focusing deviceaccording to the invention can be placed at the exit from the opticalfiber, which transmits the beam produced by laser radiation. In allcases, the focal point can easily be modified by a translationalmovement of one of the elements of the focusing device with respect tothe other. The means to be implemented are thus not very large.

FIG. 8 shows a plate of a Fresnel element used within this invention. 8Fresnel pupils are clearly shown, each having a diameter of 20 mm. Theyare distributed on a 127 mm (5 inch) diameter disk, the geometric centerof each pupil being located at 50 mm from the center of the disk.

FIG. 9 shows a plate of a portion of a Fourier diffractive element usedfor the invention. The plate is made with an electronic microscope, at ascale of 3 cm to 10 μm.

We will now describe an embodiment of the invention for a cutting ordismantling device, with reference to FIG. 10. Reference 92 denotes alaser source, for example a power Nd:YAG source, usable in pulse mode,and for which pulses may be controlled by frequency, energy and durationparameters. An optical fiber 94 is used to transport radiation outputfrom the pulsed laser source to the part to be cut 96, located at adistance. In the case of dismantling operations, this distance may be ashigh as about 10 meters. The beam 100 is then projected towards the part96, the end of the optical fiber possibly being held for example in acutting head 98. An optical focusing device such as that described above(and not shown in FIG. 10) is used to focus the laser beam 100 onto animpact area 102, close to the surface of the part to be cut. Thisfocusing device, with a fixed or variable focal length, may beincorporated into the cutting head 98. Furthermore, means 104, forexample comprising a camera and/or telemeter and/or profile meter, maybe provided close to head 98, or at the end of optical fiber 94, inorder to evaluate the distance to the surface of the part.

Data transmitted from means 104 may be analyzed in a control device 106provided for this purpose. A pulse control device (frequency, duration,energy, etc.) of the laser source 92 can be provided, consequently itmay be integrated in the control device 106. This device may for examplecomprise a conventional microcomputer or microprocessor suitablyprogrammed to analyze data. Appropriate program instructions may berecorded on a magnetic disk or on conventional RAM or ROM type drives.Furthermore, means 108 of displaying analyzed data or the imagedisplayed using the camera may also be provided, and an operator caninput control data using a keyboard 109 or a control station. Theoperator can thus make a decision about whether or not it is necessaryto move the end of the optical fiber, and/or the focusing device and/orthe focal length, with respect to the surface of part 96.

Means for controlling the position of the end of the fiber and/or meansof controlling the beam focusing may also be provided; in particular,the cutting head and/or the focusing device may be automaticallydisplaced whenever a certain distance, for example between the cuttinghead and the surface of the part 96 to be cut, has been measured andwhen comparison means, or the operator, has determined that thedifference between this measured or evaluated distance and a certainpredetermined distance, for example previously recorded in thememorization devices mentioned above, exceeds a certain distance.

Furthermore, it is often preferable to move the cutting head withrespect to the part, rather than moving the part with respect to thecutting head, in order to make a cut groove. The part is often large,for example it may be part of a nuclear installation. In this case,means may be provided for the operator to control movements of thecutting head along a trajectory. Thus in the example shown in FIG. 10, acutting head 98 includes the end of the fiber, and this cutting head ismoved with respect to the part by means of a robot controlled arm 110enabling various displacements in space (translation, pivoting aboutsome axes). A robot-controlled arm 110 can be remote controlled usingthe control console 106 and interactive control means 108, 109. In thistype of control, it may also be useful to be able to vary the headcutting speed with respect to the part to be cut.

In the case of a fixed focus, the remote operated arm 110 may be locatedin a cell to be dismantled. Tolerances on the long focal length f of thefocusing device enable it to cut the environment to f±Δf, where Δfdepends on the focal length and the characteristics of the materials tobe cut.

Targets may be identified and automatically followed by computer inputand shape analysis devices, or semi-automatically by including anoperator in the loop, using resources of the remote presence devicesparticularly by means of methods developed for computer aided remoteoperation. These methods may be installed in the microcomputer of thecontrol device 106.

The use of a “zoom” operating by displacement of one of the diffractiveelements coaxially with respect to the other can result in a largeworking area varying from 0 m to 10 m. For longer distances, beamdisturbances related to the optical trajectory in air have to be takeninto account.

The use of the “zoom”, for which the focal length can be servocontrolledstarting from a measurement of the profile of the part to be cut, makesit possible to fire at variable distances, but also to relieve theremote operated arm 110 of the “hold at constant distance” from the partto be cut function.

The assembly forms homogeneous and efficient instrumentation, since itis compact and less sensitive to environmental disturbances (vibrations,shocks, dust, etc.), including ionizing radiation, than any other knowntechnologies; therefore, the assembly is genuinely adapted to use onsite, for example on nuclear power station dismantling sites. Thefocusing system making use of digital diffractive optics, is verylightweight (of the order of 250 g) and may easily be replaced in theform of a module; due to specific assembly features, no opticalpositioning and realignment is necessary. Maintenance operations on thecutting device may be simplified, since wear parts may be easilyreplaced by a standard module exchange; for example, a flash lamp forthe laser pump or optical fiber to be replaced every 5000 hours of useby a standard exchange. The ends of removed fibers, if they are notexcessively irradiated, may be repolished and reinstalled 2 or 3 timesfor another 5000-hour cycle.

What is claimed is:
 1. An optical device, comprising: a first opticalelement configured to diffract and separate an incident beam into nbeams along n directions symmetric in rotation about an optical axisdefined by the incident beam, n being an integer; a second opticalelement comprising n Fresnel elements configured to focus the n beamsonto a focal point on the optical axis, each Fresnel element beingassociated with one of the n beams.
 2. An optical device according toclaim 1, wherein: the first optical element is configured to reflect theincident beam into said n beams, and each of said Fresnel elements isconfigured to reflect and focus each of said n beams onto said focalpoint.
 3. An optical device according to claim 2, further comprising acollimation element configured to collimate the incident beam.
 4. Anoptical device according to claim 2, further comprising a displacementmechanism configured to apply relative displacements of the firstoptical element with respect to the Fresnel elements.
 5. An opticaldevice according to claim 2, further comprising: a laser configured togenerate a laser beam; and a transmission mechanism configured totransmit said laser beam to said first optical element.
 6. An opticaldevice according to claim 1, wherein: the first optical element isconfigured to transmit said incident beam into said n beams, and each ofsaid Fresnel elements is configured to transmit and focus each of said nbeams onto said focal point.
 7. An optical device according to claim 6,further comprising a collimation element configured to collimate theincident beam.
 8. An optical device according to claim 6, furthercomprising a displacement mechanism configured to apply relativedisplacements of the first optical element with respect to the Fresnelelements.
 9. An optical device according to claim 1, further comprisinga collimation element configured to collimate the incident beam.
 10. Anoptical device according to claim 9, wherein the collimation elementcomprises a refractive element.
 11. An optical device according to claim10, further comprising a displacement mechanism configured to applyrelative displacements of the first optical element with respect to theFresnel elements.
 12. An optical device according to claim 9, furthercomprising a displacement mechanism configured to apply relativedisplacements of the first optical element with respect to the Fresnelelements.
 13. An optical device according to claim 1, further comprisinga displacement mechanism configured to apply relative displacements ofthe first optical element with respect to the Fresnel elements.
 14. Anoptical device according to claim 1, further comprising: a laserconfigured to generate a laser beam; and a transmission mechanismconfigured to transmit said laser beam from said laser to said firstoptical element.
 15. An optical device according to claim 14, whereinsaid transmission mechanism comprises an optical fiber.
 16. An opticaldevice according to claim 15, wherein said optical fiber has an endturned towards a part to be cut.
 17. An optical device according toclaim 16, further comprising an evaluating device configured to evaluatethe distance between the end of the optical fiber and the part to becut.
 18. An optical device according to claim 15, further comprising acontrolling mechanism configured to control at least one of a positionof the end of the fiber and a beam focus.
 19. An optical deviceaccording to claim 14, wherein said Fresnel elements are configured torefocus said n beams onto a part so as to cut said part.
 20. An opticaldevice according to claim 19, further comprising a triggering mechanismconfigured to trigger a laser beam emission, in a pulsed and relaxedmanner.
 21. An optical device according to claim 1, wherein n is greaterthan two.
 22. An optical device according to claim 1, wherein n is equalto
 4. 23. An optical device according to claim 1, wherein n is equal to6.
 24. An optical device according to claim 1, wherein n is equal to 8.25. An optical device according to claim 1, wherein n is equal to 10.